1. Field of the Invention
This invention relates to electro-optic voltage measuring apparatus in which a shift in phase of polarized light passed through a crystal exhibiting birefringence in proportion to an applied electric field provides a measure of the voltage producing the field. More particularly, the invention relates to such apparatus provided with temperature compensation.
2. Background Information
Electro-optical systems for measuring electric voltages are known. For instance, devices known as Pockel cells utilize certain crystals which exhibit birefringence, that is a difference in the index of refraction in two orthogonal planes, in the presence of an electric field. Some of these crystals, such as, for example, KDP (potassium dihydrogen phosphate), have a fourfold axis of rotary inversion. Such materials have the property that in the absence of an electric field the index of refraction for light propagating along the fourfold axis is independent of the plane of polarization of the light. However, if an electric field is applied parallel to the direction of the light, the index of refraction for light polarized in one direction transverse to the fourfold axis, known as the fast axis decreases and that in an orthogonal direction, also transverse to the fourfold axis, and known as the slow axis, increases by an amount which is proportional to the strength of the electric field. In such Pockel cell devices, if light is polarized in a plane which forms an angle to these transverse axes, the component of the polarized light in the direction of the slow axis with the increased index of refraction is retarded with respect to the other component. If the crystal is aligned with its fourfold axis extending between the objects between which the voltage is to be measured is applied, and the polarized light is directed parallel to the fourfold axis, the total retardation will be proportional to the total voltage differential between the two objects. This retardation is typically measured in wavelengths. The retardation is detected in an analyzer and converted to an electrical signal for producing an output representative of the magnitude of the voltage generating the field. Due to the cylic nature of this electrical signal, the output is only unambiguous for voltages producing a retardation which is less than the halfwave voltage for the crystal. In KDP, this halfwave voltage is about 11,300 volts.
U.S. Pat. No. 4,904,931 discloses an electro-optic voltage measuring system in which two beams of polarized light with the fast and slow components of one beam retarded with respect to the corresponding components of the other by an additional 1/4 wave over the retardation resulting from the field generated by the applied voltage, are passed through the electro-optic crystal and converted into two electrical signals in quadrature which are used to generate the instantaneous value of an applied voltage of any magnitude. One application of the voltage measuring system of U.S. Pat. No. 4,904,931 is in measuring the very high voltages present in electric power generation, transmission and distribution systems.
These electro-optic voltage measuring systems have many advantages. They provide good electrical isolation from the voltage being measured. Through the use of optical fibers, it is possible to easily and conveniently provide remote indicators which are not subject to the electrical disturbances which remote indicators fed by electrical signals must contend with.
However, the outputs of electro-optic voltage measuring systems constructed from materials having a fourfold axis of rotary inversion exhibit a temperature coefficient. In some of these materials, such as KDP, the temperature coefficient reaches approximately 0.3 percent per degree centigrade. Such a value of the temperature coefficient precludes the use of these materials in electro-optic voltage measuring systems designed for accurate measurement of voltages without temperature compensation. Even in materials not exhibiting such a high temperature coefficient, temperature compensation can improve the accuracy of the voltage measurement.
It is the primary object of the present invention therefore, to provide an electro-optic voltage measuring system utilizing an electro-optic crystal having improved accuracy.
More particularly, it is an object of the invention to provide such a voltage measuring system with temperature compensation.
It is a further object of the invention to provide such a voltage measuring system which accommodates for leakage and fringing of the electric field produced by the voltage being measured.